The invention relates to a detector circuit intended to produce an output related to the peak to peak signal input and that can be directly coupled. That is, no d-c blocking capacitor is needed for coupling to the signal source. The circuit is intended for use in optical disc applications where the signal to be processed has an a-c envelope that must be peak to peak detected and may contain a superimposed d-c level. Our copending application Ser. No. 619,965, filed June 12, 1984, is titled MATRIX CIRCUIT FOR OPTICAL DISC SYSTEMS and is assigned to the assignee of the present invention. This copending application discloses some optical disc circuitry and its teaching is incorporated herein by reference. The main application of the present invention is in drop out detection wherein a loss of signal must be sensed and flagged. In the drop out state the input signal peak to peak value falls below some predetermined value thereby showing a system failure.
In the optical disc system light is reflected from a modulated spiral record track and focused upon a photodetector. The photodetector output has a d-c value related to average light value and a superimposed a-c data signal. The a-c signal component can have its value vary in terms of the negative peaks, the positive peaks, or both. Therefore, it is necessary to sense the peak to peak signal in order to determine the signal quality. Desirably this will be accomplished in the presence of the d-c signal average value which must not interfere with the detection function.